1. Field of the Invention
The present invention relates to an image sensor including a plurality of photodiodes, and to a method for forming an isolation structure for photodiodes provided in an image sensor.
2. Description of Related Art
Image sensors typically include a plurality of photodiodes, and are generally capable of determining a light amount distribution with respect to the arrangement of the photodiodes. The photodiodes are each adapted to generate photovoltaic power according to the amount of received light.
In order to determine the amounts of light received by the respective photodiodes in such an image sensor, electric currents caused by the photovoltaic power generated by the respective photodiodes should independently be extracted. To this end, an isolation portion is provided between each two adjacent photodiodes to prevent an electric current from flowing between the adjacent photodiodes.
Japanese Unexamined Patent Publication No. 2000-312024 discloses an image sensor which has an isolation portion including a LOCOS (localized oxidation of silicon) isolation region and a P-type region provided on an N-type silicon semiconductor substrate. The image sensor is constructed so that a reverse bias voltage is generated between the P-type region and a semiconductor region or the semiconductor substrate to prevent an electric current from flowing between the adjacent photodiodes.
However, where the P-type region of the isolation portion is formed by diffusion of an impurity, a carrier is generated by light incident on the isolation portion, and flows into the adjacent photodiodes. Therefore, the amounts of light received by the respective photodiodes cannot accurately be determined. To avoid this problem, the area of the isolation portion is sufficiently increased for suppression of the flow of an unwanted electric current. However, this increases the area of each of the photodiodes, making it impossible to increase the integration density of the photodiodes. If the size of the photodiode is to be kept unchanged, the light receiving area of the photodiode is reduced. That is, the proportion of the total light receiving area of the photodiodes on the semiconductor substrate is reduced.
On the other hand, Japanese Unexamined Patent Publication No. 9-213917 (1997) discloses an image sensor which includes a dielectric isolation region formed by filling polysilicon in a trench having an oxidized interior surface for preventing an electric current from flowing between adjacent photodiodes.
However, there is a possibility that semiconductor surfaces of the photodiodes have defects. Therefore, where a trench (dielectric isolation region) having a smaller width is provided alone as the isolation portion, it is impossible to reduce a leak electric current occurring in the semiconductor surfaces of the photodiodes.
Further, where the isolation portion of either of the aforesaid types is provided, a depletion layer is liable to spread into the defective semiconductor surfaces It is impossible to prevent the influence of the depletion layer on the adjacent photodiodes, resulting in variations in the electrical characteristics of the image sensor.
With the provision of the isolation portion defined by the trench, carriers are liable to be trapped by defective potentials present around the trench to remain as residual charges. The residual charges thereafter influence the amounts of the electric currents generated by the respective photodiodes. Where the photodiodes of the image sensor first read a bright portion of an original image and then a dark portion, for example, carriers occurring during the reading of the bright portion partly remain as residual charges. When the dark portion is thereafter read, the electric currents (output signals) generated by the photodiodes are varied by the residual charges. This makes it impossible to accurately measure light amounts for the dark portion.